This invention relates to digital data storage and retrieval and, more particularly, an optical data storage and retrieval system and method.
Recordable/erasable optical disks are currently available for use as data storage media. Magneto-optical recording is the technique commonly used to store the data on and/or retrieve the data from the disk. During recording, a magnetic field orients the polarity of a generalized area on the disk, while a laser pulse heats a localized area thereby fixing the polarity of the smaller area. The localized area with fixed polarity is commonly called a pit. Some encoding systems use the existence or absence of a pit on the disk to define the recorded data as a 1 or 0, respectively. The most commonly used encoding system for this pit-type recording is the run length limited (RLL) 2,7 code because it gives the highest data-to-pit ratio. This type of recording, however, does not lead to higher density because amplitude and timing margins deteriorate very rapidly-as frequency is increased.
According to a feature of the invention, a method for storing and retrieving digital data on an optical disk is provided. For writing on the disk, a binary signal having first and second binary values at a given clock interval is generated. Energizing pulses that have a duration less than the clock interval are generated during each clock interval having one of the binary values. The energizing pulses turn a laser beam on and off depending on the value of the binary signal. The laser beam is focused on a recording surface of a rotating disk such that the laser beam can selectively access one of a plurality of concentric or spiral tracks on the recording surface.
For reading data on a disk, a focused laser beam is directed at the recording surface of a rotating optical disk such that the laser beam can selectively access one of a plurality of tracks on the recorded surface. The rotation of the laser beam reflected off of the recorded surface is detected by means of Kerr rotation. A change in Kerr rotation to the first type represents the first binary value. A change in Kerr rotation to the second type represents the second binary value. A binary signal that represents the binary values that occur at a clock interval generated from the occurrences of the binary values and changes in rotation occurring at boundaries of the clock interval is generated.
An apparatus for recording data on an optical disk is provided. A source originates digital data at a given clock interval. A circuit converts the data to a binary signal having first and second binary values at a given clock interval and representative of the data. A pulse generator produces energizing pulses having a duration less than a clock interval for converted data having the first binary value. A laser controller applies the energizing pulses to a laser to turn the focused laser beam on and off responsive to the energizing pulses. A circuit directs the laser at a recording surface on a rotating optical data storage disk, such that the laser beam can selectively access one of a plurality of tracks on the recording surface.
An apparatus for reading data on an optical disk is provided. A circuit rotates an optical data storage disk having a recorded surface. A controller directs a laser with a focused beam at the recorded surface such that the laser beam can selectively access-one of a plurality of tracks on the recorded surface. A circuit detects the rotation of the laser beam reflected off of the recorded surface. A decoder converts changes in rotation of a first and second type into first and second binary values, respectively. A circuit generates a binary signal that represents the binary values that occur at a clock interval generated from the occurrences of the binary values and changes in rotation occurring at boundaries of the clock interval.
An optical data storage and retrieval system is provided with data recording and reading utilizing cell boundary transition codes, special signal processing, and control of write pulses. Specifically, a data encoder encodes digital data in a code where transitions occur only at cell boundaries. A cell is defined as the encoded bits that represent one data bit. For example, in RLL 2,7, bits can be recorded at the cell boundary or at the center of the cell. The detection window is +/xe2x88x9225% of a cell. Preferably in this invention, a pulse group code recording (GCR) 8/9 code is used. Because eight data bits are encoded into nine bits, a cell is effectively defined as one data bit. For this invention, a cell will occur at each clock interval. The detection window becomes +/xe2x88x9250% of the cell. Furthermore, the GCR 8/9 code allows a limited number of consecutive zeros, e.g., three, even across word boundaries. This code also contains self clocking.
With certain data patterns, the timing margin can be enhanced. A monitor that looks for data sequences which match these predetermined data patterns is included. When one of these data patterns occurs, the laser is pulsed earlier, preferably 4 to 6 nanoseconds. Under normal writing, the laser is pulsed uniformly. With some data patterns an increase in effective write power creates better defined edges. A second monitor looks for these data sequences and when one occurs, the laser will not be pulsed off as it would under other data patterns. Thus, the effective write power is increased.
To reduce the asymmetry of the rise and fall of an isolated pulse, signal processing will be performed for reshaping the read data waveform. For example, the pulse will be narrowed and amplified. The preferred embodiment differentiates the amplified read waveform. The amplified signal is summed with its derivative resulting in a narrowed and symmetrical pulse.
For lower frequency signals, the pulse slimming will produce overshoot. Because this overshoot is predictable, the threshold of the read circuitry can be increased momentarily to prevent false data reads. A monitor will monitor the reshaped waveform, and, upon the occurrence of an overshoot, the monitor will increase the threshold of the read waveform detector.
According to another feature of the invention, an optical data storage and retrieval system is provided with downward compatibility from a high-density recording format to a low-density, ANSI format. Specifically, a first write encoder encodes digital data in a first, preferably high-density format. A second write encoder encodes digital data in a second (i.e., ANSI) format. A first read decoder decodes digital data from the first format. A second read decoder decodes digital data from the second format. A disk drive receives a 90 millimeter replaceable optical disk. A read/write head reads encoded data from and writes encoded data to a 90 millimeter optical disk received by the drive. In a first mode, the first encoder is connected between a source of digital data and the read/write head, and the first decoder is connected between the read/write head and the utilizing apparatus. In a second mode, the second encoder is connected between the source and the read/write head, and the second decoder is connected between the read/write head and the utilizing apparatus. Control electronics switch between the first and second modes, depending upon the format in which data is recorded on the disk received by the disk drive. As a result, the system can exploit high-density recording formats, while achieving downward compatibility to the low-density, ANSI format. Thus, data can be stored and retrieved in different formats in a single system that employs the same read/write head and disk drive.
According to yet another feature of the invention, the first and second formats are organized into sectors having the same number of bytes, there being more sectors in the first format than in the second format in accordance with the higher density. As a result, the same interface electronics can be employed to store and retrieve data in both formats.